In thermally assisted magnetic recording wherein a magnetic recording medium is irradiated with an evanescent light whereby the surface of the medium is locally heated and the coercive force of the medium is decreased, and writing is effected. This thermally assisted magnetic recording attracts attention as a magnetic recording system of the next generation which is capable of realizing a high plane recording density of approximately 1 T bit/inch2 or larger. In the case when the thermally assisted magnetic recording system is adopted, even when a magnetic recording medium having a coercive force of several tens kOe at room temperature is used, writing can easily be effected with a magnetic head having the currently available magnetic recording field.
Therefore a magnetic material exhibiting a high magneto crystalline anisotropy Ku of higher than 106 J/m3 can be adopted for the recording layer. Thus, average particle diameter of magnetic crystal grains can be reduced to 6 nm or smaller while a high thermal stability is maintained. As such high Ku material, there can be mentioned, for example, a FePt alloy with an L10 type crystalline structure having an approximately Ku of 7×106 J/m3 and a CoPt alloy with an L10 type crystalline structure having an approximately Ku of 5×106 J/m3.
In the case when a FePt alloy with an L10 type crystalline structure is used for the magnetic layer, the FePt alloy crystal grains in the layer must be (001)-ordered. It is preferable that this magnetic FePt alloy layer with an L10 type crystalline structure is formed on a (100)-orientated MgO-containing underlayer. The (100) plane of MgO exhibits good lattice constant conforming with the (001) plane of L10 type FePt alloy. Therefore, when the magnetic FePt alloy layer with an L10 type crystalline structure is formed on the (100)-orientated MgO-containing underlayer, the resulting magnetic layer exhibits (001)-orientation.
To decrease a media noise and enhance an S/N ratio of the magnetic recording medium, the particle diameters of magnetic crystal grains must be rendered fine even in the thermally assisted magnetic recording medium. For this purpose of rendering fine the magnetic crystal grains, it is effective to incorporate an oxide such as SiO2 or TiO2 as a grain boundary segregation material in the magnetic layer. That is, FePt crystal grains can be of a granular structure such that the crystal grains are surrounded by the added oxide such as SiO2.
The particle diameters of magnetic crystal grains can be rendered fine by adding an increased amount of the grain boundary segregation material. For example, it is described in J. Appl. Phys. 104, 023904, 2008 that the particle diameters of FePt alloy magnetic crystal grains can be reduced to 5 nm by adding 20 volume % of TiO2. Further, it is described in IEEE. Trans. Magn., vol. 45, 839, 2009 that the particle diameters of FePt alloy magnetic crystal grains can be reduced to 2.9 nm by adding 50 volume % of SiO2.